Linear integrated circuits often require a stable voltage reference that does not change substantially with temperature, operating voltage, or run-to-run resistor variations. In many cases, Zener-referenced bias circuits generate too much noise to be useful. Since sources that are referenced to the base-emitter voltage (Vbe(on)) and the threshold voltage (V.sub.t) have opposite temperature coefficients TC.sub.f, it is possible to construct a circuit that references its output voltage to a weighted sum of Vbe(on) and V.sub.t. By proper weighting, a near zero temperature coefficient TC.sub.f can be attained. Voltage variations of less than 50 ppm/.degree. C. over the military temperature range of -55.degree. C. to 125.degree. C. can be obtained. This class of reference circuits is normally referred to as band-gap references because the output voltage level at which zero TC.sub.f occurs is approximately equal to the band-gap of silicon. The mathematical derivation of this value can be found in the book "Analysis and Design of Integrated Circuits" by Paul R. Gray and Robert G. Meyer.
Prior implementations of the band-gap reference have taken several forms. One of the simpler forms utilizes a feedback loop to establish an operating point in the circuit such that the output voltage is equal to a Vbe(on) plus a voltage proportional to the difference between two base-emitter voltages. The operation of the feedback loop will be described in more detail later. However, it should be noted here that this type of band-gap reference has three stable operating points. If the circuit is to be operated in high transient radiation environments, then one must be concerned with the possibility of transient radiation induced photocurrents flipping the circuits to one of the other two stable operating points. Special "startup" circuitry is typically used to constrain the gain loop of the circuitry to operate at the desired stable operating point. However, the possibility still exists that transient radiation will cause this type of circuitry to switch to the second (undesired) stable operating point. Another problem with this known reference circuit is that the current on which the voltage reference is based is derived from the power supply and therefore may vary with power supply variations.
Another band-gap reference circuit is known that is essentially independent of supply variations. This known circuit will be described in more detail later. For now, it is sufficient to note that this known circuit will have, under certain conditions, two stable operating points.
An object of the present invention is to provide a band-gap reference circuit which has only one stable operating point. Such a circuit needs to meet voltage regulator requirements of linear/analog circuits designed for high radiation environments. This is because band-gap reference circuits which have more than one stable operating point pose special problems in radiation environments. The possibility exists that photocurrents generated by high Gamma rate exposure could cause the circuit to switch to an undesirable operating point. There is therefore the need for a band-gap reference circuit that eliminates the need for any special start-up circuitry and provides stability in transient radiation environments.